FIG. 7 schematically illustrates a wind farm 10. A wind farm 10 may comprise one or more wind turbines 11 which are connected to a common node which may also be referred to as point of common coupling 12 to which the electric energy is provided and from where the electric energy is transferred to the utility grid 14, in particular via one or more transformers 13. The utility grid 14 may be connected via a high voltage direct current system or via a high voltage alternating current system.
There have been problems observed, when the wind farm is connected to a weak grid, i.e. a utility grid which has a small short circuit ratio or a large reactance. In particular, the weaker the grid is, the less active power may be transferable to the grid. Transferring maximum active power may require having an angle between the current and the voltage of 90°. However, a maximum operating angle is recommended in traditional power system, for example, 30° because above the certain angle the operating point may rapidly approach the critical point with an incremental change in the transmitted power, characterized by a rapid increase in dV/dQ at the receiving end.
It has been observed, that when the power transmitted approaches the angle stability limit, the voltage at a mid-section of the transmission line may be depressed and voltage collapses may be experienced. According to a conventional system, the voltage controller of the wind turbine causes the wind turbine to inject a large amount of reactive power into the grid which may boost the voltage back to a nominal value. However, the kinetic energy stored in the rotor system of the wind turbine then decreases or pumps back to the system and causing again a depressed voltage which further causes wind turbines to be tripped.
Further, it has been observed that a loss of voltage stability occurs when the wind turbine is connected to a weak grid. A low short circuit ratio may be due to a failure in a transmission line connecting the point of common coupling to the utility grid.